Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B3/00—Electrolytic production of organic compounds
  • C25B3/20—Processes
  • C25B3/25—Reduction

Definitions

• the present invention concerns a process for the preparation of p-amino phenols of the general formula I set forth in the introductory protion of claim 1, by electrolytic reduction of p-phenylazophenols in an aqueous medium, and the process of the invention is characterized by performing the electrolytic reduction in a basic medium at a pH value at least equal to the pKa value of the p-phenylazophenol and at an elevated temperature of at least 50°C and preferably about 70 to 100°C.
• the compound 5-aminosalicylic acid may be conveniently obtained, said compound being a valuable active component in certain medicaments, cf. the PCT Application 81/02671, for the treatment of colitis ulcerose and Crohn's disease.
• Ar and Ar' are optionally substituted phenyl groups
• a diazoted aromatic amine an aryldiazonium compound
• a phenol in a basic medium
• H.E. Fierz-David & L. Blangley Grundlegende Operationen der Weg, 5th ed., Vienna 1943.
• This known coupling reaction has been used for many years in the production of dyes.
• the reaction is as follows Arylazo.phe ⁇ ols can be reduced electrolyticallyy in an. acid medium to amines and amino phenols. The reaction can either take place directly (see e.g. Chem. Abstr., 13, 843 (1919) and Chem.
• the US Patent Specification 3 645 864 describes electrolytic reduction in an acid medium.
• the starting material is nitrobenzene which is reduced to p-amino phenol and its derivatives at 60 to 150°C and at a cathode potential of -0.25 to -0.35 V with respect to a saturated calomel electrode.
• electrolytic preparation of amino phenols proceeds in a basic medium, the electrolyte solution being an alkali metal hydroxide solution.
• the starting materials are nitrosophenols which must be synthesized beforehand in an inert atmosphere, and to obtain reasonable results it is necessary to use a large number of electrolysis cells in series connection.
• the present process can in principle be used for the reduction of all arylazophe ⁇ ols with the single restriction that the phenol group is para-positioned with respect to the azo group.
• the two substituents R 1 and R 2 are independently selected from among hydrogen, optionally substituted alkyl groups, halogens, COOH, SO 3 H or NO 2 ; the type of the substituents is not critical when only the substituents are not reduced under the given reaction conditions.
• the electrolysis is performed in an aqueous basic medium whose pH value is determined by the pKa of the p-arylazophenol used as the starting material.
• pH will be 8 to 10 or more, depending upon the starting material. It is believed that the reaction rate increases with increasing pH, so pH>12 is often used.
• the temperature used is sufficiently high to ensure a reasonable reaction rate. Frequently, this temperature is between 70 and 100oC, at which the reduction proceeds at a reasonably high rate. Temperatures above 100°C can also be used, but this is no advantage in terms of energy.
• the potential used is up to 0.7 V, preferably about 0.5 V more negative than the reduction potential (halfwave potential) at the given pH value. A more negative potential is not harmful, unless other groups or substances are reduced by this.
• the potential is not significantly temperature-sensitive.
• the current intensity used is the current density (A/dm 2 ) multiplied by the electrode area. The current density used depends upon the supply of reducible material, which is a function of concentration and transport conditions (laminar or turbulent flow) in the reactor.
• Preferred compounds produced by the process of the invention are p-amino phenol and 5-aminosalicylic acid.
• a third container (C) 28 kg (202 moles) of salicylic acid are dissolved in 33 litres of concentrated sodium hydroxide solution (500 g of NaOH in 1 litre solution) and 67 litres of water to which 2 kg of anhydrous sodium carbonate have been added. After cooling to 0°C, the contents are pumped slowly from the container (A) and with stirring to a container (C), so that the temperature is kept below 5°C. The azo compound gradually precipitates and finally becomes a thick porridge-like mass. The last part of the coupling proceeds slowly, and it is necessary to stir for 5 or 6 hours after completed addition of the diazo solution from the container (A).
• a concentrated sodium hydroxide solution 500 g of NaOH in 1 litre solution
• heating is performed until everything has been dissolved and pH is above 12.
• the contents are pumped into another container (D), followed by heating to 80°C.
• the contents are pumped through the electrolysis cell, which may be a "filter press cell” (SU Electro Syn Celle) with a lead cathode potential of at least -1.4 V (measured against a standard calomel electrode).
• the current density is 10 to 20 A/dm 2 . After 20000 Ah, the current density is reduced to 2 to 3 A/dm 2 , and after another 2 hours the electrolysis is stopped.
• the solution is decolored by addition of 5 kg of sodium hydrosulfite and is pumped into a container (F) blown through with nitrogen.
• the diazo compound After cooling to 0oC, the diazo compound is added slowly and with stirring, so that the temperature does not exceed 5°C.
• the resulting coupling product is a viscous mass which is stirred overnight.
• the resulting azo coumpound (0.8 mole) is admixed with a mixture of concentrated NaOH and water to dissolve the coupling product before the electrolysis.
• the pH value hereby exceeds 12.
• the produced amount of azo compound is sufficient for two electrolyses.
• Half of the solution (corresponding to 0.4 mole of 5-phenylazosalicylic acid) is poured into the cathode compartment of the electrolysis cell.
• An NaOH solution is poured into the anode compartment.
• the contents are pumped through the electrolysis cell, and the reaction is started.
• the electrolysis has terminated, the reduction product is tapped into a flask. Cooling is effected, and HCl is added to pH 4.0. After filtration the residue ( 5-aminosalicylic acid) is washed in H 2 O and acetone.
• the electrolysis is performed in a conventional electrolysis cell in which the anode compartment and the cathode compartment are separated by a semi-permeable membrane.
• the cathode is of lead, and the anode is of nickle.
• the cathode reference electrode is an Ag/AgCl electrode.
• the reference voltage must be greater than 0.8 V, which is the natural potential of the Ag/AgCl electrode. A reference voltage below this value means that there will be no reduction. The reference voltage should be as close to 1.5 V as possible and be maintained at that value in order for the reduction to proceed satisfactorily.
• example 2 owing to the relatively low temperature of 60°C, the reference voltage has only just reached 1.2 V (however not all the time). This involves an inferior reaction process, and the reaction should therefore proceed at a temperature of at least 70°C.
• the high yield of production in example 2 is probably due to the relatively great unreliability associated with the test because the substance quantities involved are very small.
• the cathode compartment is provided with a thermometer and a reflux condenser. Venting with nitrogen, and a nitrogen atmosphere is maintained in the cathode compartment during the entire reduction.
• the temparature is increased to 80°C, and electrolysis is performed at -1.2 V, measured against a standard calomel electrode, with stirring with a magnet stirrer.
• the initial current density is about 10 A/dm 2 .

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• Chemical & Material Sciences (AREA)
• Organic Chemistry (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Low-Molecular-Weight Compounds And Preparation Thereof (AREA)
• Electrolytic Production Of Non-Metals, Compounds, Apparatuses Therefor (AREA)
• Pharmaceuticals Containing Other Organic And Inorganic Compounds (AREA)

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• 1984
  • 1984-11-22 DK DK553784A patent/DK153412C/da not_active IP Right Cessation
• 1985
  • 1985-11-20 DD DD85283040A patent/DD242640A5/de not_active IP Right Cessation
  • 1985-11-21 JP JP60505292A patent/JPS62501218A/ja active Pending
  • 1985-11-21 HU HU86129A patent/HU199106B/hu not_active IP Right Cessation
  • 1985-11-21 DE DE8585905787T patent/DE3569724D1/de not_active Expired
  • 1985-11-21 WO PCT/DK1985/000108 patent/WO1986003194A1/en active IP Right Grant
  • 1985-11-21 US US06/882,921 patent/US4670112A/en not_active Expired - Lifetime
  • 1985-11-21 EP EP85905787A patent/EP0203122B1/en not_active Expired
  • 1985-11-21 ES ES549140A patent/ES8609208A1/es not_active Expired
• 1986
  • 1986-07-21 SU SU864027853A patent/SU1493101A3/ru active

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